n只会被最小质因子筛去的证明：因为从小到大枚举

#include <iostream>
#include <algorithm>

using namespace std;

const int N= 1000010;

int primes[N], cnt;
bool st[N];

void get_primes(int n)
{
    for (int i = 2; i <= n; i ++ )
    {
        if (!st[i])  primes[cnt++]=i;
        for(int j=0;primes[j]<=n/i;j++){
            st[primes[j]*i]=true;
            if (i % primes[j] == 0) break;
        }
    }
}

int main()
{
    int n;
    cin >> n;

    get_primes(n);

    cout << cnt << endl;

    return 0;
}